Exaggerated sexual swellings and male mate choice in primates: testing the reliable indicator hypothesis in the Amboseli baboons

Abstract

The paradigm of competitive males vying to influence female mate choice has been repeatedly upheld, but, increasingly, studies also report competitive females and choosy males. One female trait that is commonly proposed to influence male mate choice is the exaggerated sexual swelling displayed by females of many Old World primate species. The reliable indicator hypothesis posits that females use the exaggerated swellings to compete for access to mates, and that the swellings advertise variation in female fitness. We tested the two main predictions of this hypothesis in a wild population of baboons (Papio cynocephalus). First, we examined the effect of swelling size on the probability of mate-guarding ('consortship') by the highest-ranking male and the behavior of those males that trailed consorshipts ('follower males'). Second, we asked whether a female's swelling size predicted several fitness measures. We found that high-ranking males do not prefer females with larger swellings (when controlling for cycle number and conception) and that females with larger swellings did not have higher reproductive success. Our study-the only complete test of the reliable indicator hypothesis in a primate population-rejects the idea that female baboons compete for mates by advertising heritable fitness differences. Furthermore, we found unambiguous evidence that males biased their mating decisions in favor of females who had experienced more sexual cycles since their most recent pregnancy. Thus, rather than tracking the potential differences in fitness between females, male baboons appear to track and target the potential for a given reproductive opportunity to result in fertilization.

Keywords: Papio cynocephalus; male mate choice; reliable indicator hypothesis; sexual swellings.

Figure 1

Expected probability of being consorted by the highest ranking male at any time during the five day window prior to d day a) as a function of the average length of the queue, b) as a function of cycles since resumption, and c) as a function of maximal swelling size. Each trend line was generated using coefficients generated from a bivariate logistic regression with single predictor. Observed data points plotted and jittered to facilitate visibility.

Figure 2

Proportion of consort time with a follower male as a function of a) cycles since resumption and b) maximal swelling size. The y axes are the residuals from a) a linear regression of proportion of time with a follower on swelling width and swelling length and b) a linear regression of proportion of time with a follower on cycles since resumption. 2b) Closed circles represent swelling width (solid regression line) and the open circles represent swelling length (dashed regression line).

Figure 3

Heuristic illustrating the hypothesized cost-benefit relationships that would result in the pattern observed in this study. A) Male behavior with respect to cycles since resumption; if the benefit to a male of monopolizing a cycle with a higher cycle number is higher than the costs of either maintaining the consortship or engaging in following behavior, then high-ranking males should work harder to obtain those consortships and follower males should invest in following them. Cost functions are increasing because the costs of both male behaviors should increase with cycles since resumption, due to increased presence of followers as cycles since resumption increases. B) Male behavior with respect to swelling size; if the benefit to a male of monopolizing a cycle with a larger swelling size (for a given cycle number) is higher than the cost of engaging in following behavior, but lower than the cost of maintaining the consortship, then high-ranking males should not incur the costs of obtaining those consortships, but follower males should still invest in following them. Cost functions are increasing because the costs associated with both male behaviors should increase due to increased presence of followers as swelling size increases.